Lvs Scheduling Overview - Red Hat ENTERPRISE LINUX AS 2.1 Installation Manual

Section 6.3:LVS Scheduling Overview 105
This configuration is ideal for busy FTP servers, where accessible data is stored on a central, highly
available server and accessed by each real server via an NFS exported directory or Samba share. This
topography is also great for websites that access a central, highly available database for transactions.
And using an active-active configuration with Red Hat Cluster Manager, the administrator can con-
figure one high-availability cluster to serve both of these roles simultaneously.
The third tier in the above example does not have to use Red Hat Cluster Manager, but failing to use
a highly available solution would introduce a critical single point of failure. For more information on
setting up Red Hat Cluster Manager high availability cluster refer to the Red Hat Cluster Manager
Installation and Administration Guide.

6.3 LVS Scheduling Overview

One of the advantages of using an LVS cluster is its ability to perform flexible, IP-level load balancing
on the real server pool. This flexibility is due to the variety of scheduling algorithms an administrator
can choose from when configuring a cluster. LVS load balancing is superior to less flexible methods,
such as Round-Robin DNS where the hierarchical nature of DNS and the caching by client machines
can lead to load imbalances. Also, the low level filtering employed by the LVS router has advantages
over application-level request forwarding because balancing loads at the network packet level causes
minimal computational overhead and allows for greater scalability.
Using scheduling, the active router can take into account the real servers' activity and, optionally, an
administrator-assigned weight factor when routing service requests. Thus it is possible to create a
group of real servers using a variety of hardware and software combinations and the active router can
load each real server evenly.
The scheduling mechanism for an LVS cluster is provided by a collection of kernel patches called IP
Virtual Server or IPVS modules. These modules enable L4 switching, which is designed to work
well with multiple servers on a single IP address.
To track and route packets to the real servers efficiently, IPVS builds an IPVS table in the kernel. This
table is used by the active LVS router to redirect requests from a virtual server address to and returning
from real servers in the pool. The IPVS table is constantly updated by a daemon called ipvsadm —
adding and removing cluster members depending on their availability.
6.3.1 Scheduling Algorithms
The form the IPVS table takes depends on the scheduling algorithm the administrator chooses for any
given virtual server. To allow for maximum flexibility in both the types of services you can cluster
and how these services are scheduled, Red Hat Enterprise Linux AS 2.1 provides the eight scheduling
algorithms listed below. For instructions on how to assign scheduling algorithms see Section 9.6.1,
The Subsection.
VIRTUAL SERVER
Round-Robin Scheduling